Field of the Invention
The present invention relates to a sheet processing apparatus configured to bind a bundle of sheets and an image forming apparatus including the same.
Description of the Related Art
Hitherto, some image forming apparatuses such as a copier, a laser beam printer, a facsimile machine, and a multi-function printer are provided with a sheet processing apparatus configured to perform such processes as stapling on sheets on which images have been formed. Such a sheet processing apparatus is configured to bind a bundle of sheets by using a metallic staple in general. Lately, however, as another method for binding sheets, there is proposed a method of fastening a sheet bundle without using any metallic staple by considering environmental issues by entangling fibers of the sheets by biting the sheet bundle by concavo-convex teeth and forming concavo-convex dents on the sheets as disclosed in Japanese Patent Application Laid-open No. 2010-189101 for example.
However, the sheet processing apparatus described above configured to fasten the sheet bundle by biting the sheet bundle by the concavo-convex teeth has a drawback that although the sheet processing apparatus endows the sheet bundle with a predetermined fastening power in a direction in which the fibers are entangled, the fastening power drops extremely in a direction orthogonal to the direction in which the fibers are entangled. The sheet processing apparatus also has another drawback that it can fasten the sheets only with an extremely low fastening power in fastening the sheet bundle by entangling the fibers if moisture of the sheets is low or smoothness of surfaces of the sheets is high and it is hard to entangle the fibers with each other.